I'd like to record high def video of solo acoustic fingerstyle guitar, with high quality audio. I have a Sony SR 12 which provides great video quality, but the audio is not so great and the audio options are next to none. I have several top quality condenser mics that I would like to use. Tapeless is great and it has spoiled me, so I have not sprung for a camera like the Canon XH A, which would allow xlr connection of the mics. I've been waiting for a tapeless version of that camera or similar, but it doesn't look like that's happening anytime soon.

Is there a solution that's simpler than recording the audio separately and then attempting to sync it to the video in post? I'm not even sure how to do that.

I assume the mics you already have have an XLR output jack. If this is the case, you could get one of the portable pre-amp or audio adapter boxes whose main function is to convert balanced audio input (via XLR input sockets) and output the signal via their built-in 3.5mm mini jack which plugs into your camera's mic input. JUICEDLINKS and BEACHTECKS are two major manufacturers of such boxes. Most of these boxes have at least two XLR input sockets, one for each channel of audio whose levels can be controlled separately. Some can provide power to your mics if your mics require phantom power. Some can't but these are what you need if your mics are already battery-powered. All of them are designed to mount directly on the camera, underneath via the tripod socket.

If your mics have mini jack output, find a mic holder with shockmount (to minimize noise and vibration) that can be mounted on a standard coldshoe or bracket (of various designs) that mounts on the tripod socket of your camera. Best is mounting the mic normally on a mic stand and wire it to the camera.

Your option without using an external audio recorder will be limited to riding the camera's auto gain at different input levels from one of those XLR pre-amp boxes. You could control left/right channel separately from there but you still can't control the exact levels of BOTH channels at your camera's end because of the auto gain.

A few low-cost compact consumer HD camcorders offer manual audio level control which will make life easier for you as I have one of those myself. If you want to stick with your SR12 I'm afraid your only solution is to go with an external recorder with the additional costs and hassles in post.